CVE-2026-119196: pyasn1 has quadratic CPU consumption in constructed BIT STRING decoding — Fixed versions

Summary

## Summary In pyasn1 through 0.6.4, decoding a constructed BIT STRING with many fragments repeatedly shifts and copies the entire value accumulated so far. For fixed-size fragments, CPU time grows quadratically with the encoded input size. An application that passes sufficiently large, attacker-controlled ASN.1 data to an affected decoder can suffer CPU exhaustion during `decode()`; no conversion or formatting of the decoded value is required. The BER and CER decoders reach this path for constructed BIT STRING values, with or without an `asn1Spec`. The DER decoder rejects constructed BIT STRING in schema-less decoding, but its schema-driven path accepts a **definite-length** constructed BIT STRING when given a matching `asn1Spec` and reaches the same vulnerable code. Constructed BIT STRING is not valid DER; this DER exposure is due to decoder permissiveness. DER rejects the indefinite-length form. This is a separate decoder path from the OBJECT IDENTIFIER accumulation fixed in [CVE-2026-59885](https://github.com/advisories/GHSA-8ppf-4f7h-5ppj). ## Technical details In the 0.6.4 [`BitStringPayloadDecoder`](https://github.com/pyasn1/pyasn1/blob/v0.6.4/pyasn1/codec/ber/decoder.py#L173-L301), both the [definite-length](https://github.com/pyasn1/pyasn1/blob/v0.6.4/pyasn1/codec/ber/decoder.py#L237-L255) and [indefinite-length](https://github.com/pyasn1/pyasn1/blob/v0.6.4/pyasn1/codec/ber/decoder.py#L275-L301) fragment loops call `BitString.fromOctetString(..., prepend=bitString)` for every fragment. [`fromOctetString`](https://github.com/pyasn1/pyasn1/blob/v0.6.4/pyasn1/type/univ.py#L614-L627) shifts the growing `SizedInteger` left by the fragment's bit length and ORs in the fragment. Shifting an ever-larger Python integer for each fixed-size fragment makes total work O(n²) for n fragments. The fragments are siblings, so the decoder's nesting-depth limit does not constrain their count; its limit on the number of length octets does not prevent the demonstrated inputs. In…

Remediation

Inventory every pyasn1 deployment, including staging.
Upgrade pyasn1 to 0.6.5.
Confirm the running version after the upgrade.

Summary

Remediation

References